Feldspar is the most widespread mineral group in Earth’s crust, making up approximately 60% of its volume and acting as a primary component in many common rocks, such as granite and basalt. These minerals are aluminum silicates that incorporate varying amounts of sodium, calcium, or potassium into their structure. Identifying feldspar visually is an important skill for understanding the composition of many different rock types. This guide details the physical properties and visual characteristics that distinguish feldspar from other minerals, allowing for accurate identification in a hand sample.
Fundamental Visual Properties
Feldspar typically displays a vitreous, or glassy, luster, though a pearly sheen is often visible on broken surfaces. This non-metallic appearance helps differentiate it from minerals with a metallic shine. The mineral’s transparency can range from opaque to translucent, and impurities often influence this quality.
Feldspar minerals have a Mohs hardness that ranges from 6 to 6.5, making them harder than glass but softer than quartz. A feldspar sample can scratch common glass, but it cannot be scratched by a steel knife. When forming well-developed crystals, feldspar typically exhibits a blocky or prismatic habit, appearing rectangular or elongated. This shape is different from the six-sided, pointed spear shape often seen in quartz crystals.
The Diagnostic Feature: Cleavage and Fracture
The most reliable way to distinguish feldspar from other common minerals, especially quartz, is by examining how the sample breaks, a property known as cleavage. Cleavage refers to a mineral’s tendency to break smoothly along flat, parallel planes of weakness. Feldspar is characterized by having two distinct cleavage planes that meet at or very close to a 90-degree angle.
When light catches these flat surfaces, they produce a distinct, mirror-like flash as the sample is rotated. This visual feature is a strong indicator of feldspar, as broken fragments tend to form shapes resembling rectangular blocks. In contrast, quartz lacks cleavage entirely, exhibiting a conchoidal fracture that results in curved, shell-like, and irregular broken surfaces.
The presence of two well-defined cleavage planes intersecting at a right angle is a structural hallmark of the feldspar group. Observing these flat, reflective surfaces provides the clearest visual evidence that the specimen is a feldspar.
Color and Variety in Feldspar Groups
Feldspar exhibits a wide spectrum of colors, which is a secondary but useful visual characteristic for identification after cleavage is confirmed. The two main groups of feldspar, Alkali feldspar and Plagioclase feldspar, often have different color associations. Alkali feldspar, or potassium feldspar (K-Feldspar), commonly appears in shades of pink, salmon, or brick-red, often due to tiny inclusions of hematite. A well-known variety, Amazonite, is recognized by its distinct bright green to bluish-green color.
Plagioclase feldspar, which is a series ranging from sodium-rich to calcium-rich, typically displays colors from white or colorless to various shades of gray. Some varieties can also be yellow, brown, or nearly black. A unique feature of Plagioclase feldspar is the appearance of fine, parallel lines called striations on its cleavage faces. These lines are caused by a phenomenon known as crystal twinning and are a reliable way to distinguish Plagioclase from Alkali feldspar, which does not exhibit this feature.
Certain Plagioclase varieties, such as Labradorite, display a striking visual effect called iridescence, where a vibrant play of colors flashes across the surface. This optical effect, often a blend of blues, greens, and golds, is caused by the interference of light reflecting off microscopic internal layers within the crystal structure.